In 7.59 my assassin's cold-sorc merc wore a Shadowtrick armor. All worked well.
I had to make it ethereal and rub it with a few liters of Oil of Skill to get the Strength requirement down to his strength, but he could wear it.
If the armor the merc wears is part of the strength he has, then removing the armor will drop his strength. Check the merc without any armor on for what strength he has, then say whether that merc can wear the Shadowtrick.
... unless this changed with 7.60 ...